An image sensor converts an optical image into an electrical signal. Types of image sensors include charge-coupled devices (CCDs) and complementary-metal-oxide-semiconductor (CMOS) image sensors. Image sensors are commonly used in digital cameras as well as other imaging devices.
Image sensors include pixels, which accumulate charge when illuminated by light. Conventionally, pixels accumulate charge in an analog circuit for a continuous period of time referred to as an exposure time. The accumulated charge is transferred to an analog-to-digital (A/D) converter (ADC), which converts the accumulated charge into a digital value for that pixel. A conventional image sensor outputs a two-dimensional (2D) array of digital values.
In CMOS image sensors, exposure of all pixels in a same row starts simultaneously and ends simultaneously. In other image sensors, exposures of all pixels may start at the same time, but end at different times.
What is common to conventional image sensors is that the digital value of a pixel is obtained as a result of A/D conversion of a charge that was accumulated during a continuous time interval of exposure.
High-dynamic-range imaging has recently been developed which takes multiple pictures at different exposures. The different exposures are then combined to produce an image.